The invention relates to terminal apparatus for a digital telecommunications system, and is especially applicable to systems capable of communicating voice and data signals over the same channel, for example Integrated Services Digital Networks.(ISDN)
In ISDN, voice and data signals are transmitted using a channel having a bit rate of, for example, 64 kBit/second. This channel extends between central office equipment and the subscriber's premises as a two-wire line. In North America basic access comprises two 64 kBit/second "B" channels for data and one 16 kBit/second "D" channel for signalling data. This access is usually referred to as "2B+D". At the subscriber's premises, the two-wire line terminates in what is known as the "U-interface" in the subscriber's network termination apparatus (NT). The network termination apparatus supports multiple terminal apparatus (TE), each connected to the network termination apparatus by what is known as an "S" or "T" or "S/T" interface. The terminal apparatus may be connected in bus or star configuration. In North America, ANSI T1.601 governs the U interface and ANSI T1.605 governs the S/T interface.
It has been recognized that it would be desirable to reduce the expensive stocks of network terminators and terminal apparatus. U.S. Pat. No. 4,905,237 by Voelzke discloses an ISDN terminal apparatus which can be configured to connect either a T-interface or a U-interface i.e. four wire and two wire respectively. The terminal can detect automatically whether it is connected to a two wire interface or a four wire interface. When connected to a T interface, Voelzke's terminal provides for other terminals of the same type to be connected by way of an additional plug connector. Conversely, when configured for a U-interface, it makes the T-interface available at an additional plug connector to allow connection of other terminal apparatus. Accordingly, the local terminal apparatus serves as a network terminator for the U interface and also provides an interface for additional terminals.
Voelzke focuses his discussion upon power supply and automatic detection issues. He does not discuss in detail how data is handled, specifically D-channel signalling.
In a typical ISDN system, the network terminator transmits the same 2B+D signals to all terminal apparatus. Each active terminal apparatus may be assigned an individual B channel in that signal. When transmitting, the network terminator incorporates a frame marker which is derived from the normal 8 KHz frame sync of the signal it receives at the U-interface. Likewise, the terminal apparatus will include its own frame marker in the signal it returns to the network terminator.
The minimum and maximum time delays between the transmission of the frame marker and the receipt of the incoming terminal frame markers will be fixed and hence can be offset. The distance between the network terminator and the terminal apparatus closest to it will determine the earliest time that a corresponding frame marker is received by the network terminator, i.e. the minimum time delay plus the "round trip" delay. If the network terminator were integrated with a terminal apparatus in the same enclosure, with the integrated terminal apparatus connected to the S/T interface in the usual way, the integrated terminal apparatus would be the closest terminal apparatus in the system since it would be effectively at a distance of zero feet. This would limit the system to short passive bus configurations. The longer loop lengths allowed with other configurations would not be available.